The omicron variant is now the dominant strain of COVID-19 in the United States, according to federal tracking.

Centers for Disease Control and Prevention data show that omicron accounted for more than 73% of COVID-19 cases last week, up from about 13% the week prior. The agency revised its data Monday to reflect an increase in confirmed omicron cases the week ending Dec. 11, which was initially estimated to be about 3%.

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President Joe Biden’s chief medical adviser Dr. Anthony Fauci called the variant “extraordinary” in its ability to double in transmission every two to three days. But the question of whether it will produce more severe infection is still being answered.

Early data out of South Africa reported that hospitalizations due to the omicron variant were 29% lower than in the initial wave in March 2020. One major caveat, though, is South Africa's relatively high population immunity. The country has endured three robust waves of COVID-19 in which nearly everyone was exposed to the virus whether they got severely ill or not.

COVID-19 cases continue to surge in most of the U.S., especially in the Northeast and Midwest. Delta, meanwhile, still accounts for more than 26% of cases in the U.S. and has proven to cause severe illness in unvaccinated people. Hospitals were already struggling to keep up with patients amid delta's onslaught, on top of exacerbated shortages of healthcare workers across the country.

"Omicron is supplanting delta in the sense that if you look at the percentage of omicron on top of delta [infections], but delta continues in terms of the number of cases," said Dr. Ilya Finkelstein, a molecular biologist at the University of Texas, Austin. "So it may be getting into a situation of coexisting delta-omicron."

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Health officials are imploring the unvaccinated to get the shots as soon as possible, and for all eligible adults (those who got their second Pfizer or Moderna shot over six months ago) to get their boosters. A growing set of evidence suggests that a booster dose is crucial to preventing a severe infection due to omicron. For instance, Moderna announced earlier Monday that a third half dose of its vaccine increased antibody levels against the omicron variant 37-fold in lab tests. A full 100-microgram dose, meanwhile, was even more effective, boosting antibody levels 83-fold compared to pre-boost levels.